Interstitial microwave antenna with miniaturized choke hyperthermia in medicine and surgery

ABSTRACT

The present invention relates to minimally invasive surgery techniques. It provides a method for manufacturing an antenna for percutaneous acute hyperthermia microwave applications of the monopole or dipole co-axial type provided with trap, commonly called choke, for blocking the propagation of the backwards reflecting wave towards the generator. The miniaturisation of the device allows a use minimally invasive for interstitial hyperthermia in medicine and surgery, in particular for oncology. The method of manufacturing the antenna provides a metal needle ( 1 ) for the introduction of the antenna ( 2, 3, 4 ) in the target tissue. On the external conductor ( 4 ) of the antenna ( 2 ) a metal collar ( 6 ) is connected in a predetermined position; a plastics sheath ( 5 ) is applied in order to cover the external conductor ( 2 ) in the portion between the feed ( 7 ) and the collar ( 6 ); the inner wall of the metal needle ( 1 ) wherein the antenna is inserted is then used for containing and guiding the collar ( 6 ) and the sheath ( 5 ); in particular, the collar ( 6 ) being in electrical contact with the inner wall of the metal needle ( 1 ). An antenna is thus obtained with choke of variable length and with miniaturized diameter. A thermocouple can be introduced through the choke that protrudes the directly in the “feed” zone.

FIELD OF THE INVENTION

[0001] The present invention relates to minimally invasive surgerytechniques, for interstitial, percutaneous, laparoscopic, endoscopic andintrasurgery applications in medicine and surgery, especially inoncology.

[0002] More precisely, it relates to a microwave antenna, forhyperthermia, operating from 37C up to over 100C, of the monopole ordipole co-axial type equipped with “trap”, commonly called choke, forblocking the propagation of the reflecting wave that turns back towardsthe generator.

[0003] Furthermore the invention relates to a method of construction ofsuch an antenna.

DESCRIPTION OF THE PRIOR ART

[0004] Hyperthermia in oncology is method used for over 30 years fortreatment of cancer (Hahn GM, Hyperthermia and Cancer, Plenum Press, inthe York, 1982). It consists in heating the cancer cells to obtain theirnecrosis directly or with additional use of other methods such asradiotherapy, chemotherapy or other surgery techniques.

[0005] For heating tissues, in particular for treatment of surfacelesions, firstly electromagnetic waves have been used produced by asource located out of the human body.

[0006] More recently thin appliances have been used among whichmicrowave antennas, operating between some hundreds of MHz and somethousands of MHz, typically at 2450 MHz, executed in co-axial tube, forinterstitial, percutaneous, laparoscopic, endoscopic and intrasurgeryapplications, suitable to the local treatment of deep lesions (IskanderM F & Tumeh A M, Design Optimization of Interstitial Antennas, IEEETransactions on Biomedical Engineering, 1989, 238-246).

[0007] Such antennas are usually inserted in the lesion to treat usingcatheters or metal needles, under echographic guide, TAC, NMR or othercomputerised imaging techniques. They are suitable to be used inassociation with drugs, ionizing waves and/or with surgery ablation.

[0008] These microwave antennas, normally, are manufactured using aflexible or semi-rigid co-axial tube, suitably modified at one end, forconveying microwave power into the tissues to cause hyperthermia.

[0009] In FIG. 1 an axial cross section is shown of an antenna insertedin a biopsy needle 1. The antenna, in its active portion at the right ofthe drawing, is suitably configured as radiating dipole or monopole.More precisely, 2 is the external conductor of the co-axial tube, 3 isthe dielectric layer that insulates the external conductor from thecentral conductor 4. The point indicated with 7 is the feeding point,i.e. the active portion of the antenna, commonly called “feed”, wherethe emitted power, normally, is maximum.

[0010] The isothermal surfaces that can be obtained by heating abiological tissue (not crossed by large blood vessels) with a normalantenna, that for example is made by cutting at an end a portion of theexternal conductor 2 of the co-axial tube and leaving dielectric layer 3uncovered, as described in FIG. 1, have a rotationally symmetricconfiguration. Their projection on the plane of the figure iselliptical, with a central maximum of transmission, as above said, nearfeeding point 7 of the antenna, where the distal portion of the externalconductor 2 of the co-axial tube is cut. The surface of dottedprojection 8 indicates an isothermal surface of the tissue that is beingradiated by this type of antenna in a purely theoretical case.

[0011] Actually, the impedance of the antenna is never perfectly adaptedwith that of the medium in which it operates, owing to the variation ofthe dielectric characteristics of the medium same when heating, and forother reasons connected with the guided propagation of anelectromagnetic wave. During the delivery of microwave power there isalways an backward wave that returns along the external conductor of theantenna, from the active end towards the generator, causing an backwardelongation of the heating figure. The dashed curve 9 indicates theprojection of an backwardly elongated isothermal surface correspondingto this effect. This drawback prevents from suitably concentrating theheat production near the active portion of the antenna and is a biglimit to the use of this technique.

[0012] Per overcome this drawback normally the antenna is equipped witha device, called choke, or trap, often used in radio-broadcastingantennas (see for example Reintjes J F & Coate G T, Principles of Radar,McGraw-Hill Book Company, in the York 1952, p 851) that blocks thebackwards propagation of the reflected power.

[0013] This device, indicated with 11 in FIG. 2, consists in a co-axialguiding portion, λ/4 long, being λ the wavelength of the emitted waves,obtained by arranging at an end the external conductor of the co-axialtube of the antenna, near the feed 7, a short circuit metal tube 12. InFIG. 2, 1 is the needle guide of the antenna, 2 is the externalconductor of the antenna, 3 is the insulating material and 4 is thecentral conductor.

[0014] In this case the backwards reflecting wave, indicated with 13,runs the external surface of the antenna, enters choke 11, reflectsitself at its end in a short circuit and, after a total path of λ/2 isagain at the entrance of the choke but in phase opposition with respectto that that at inlet, with a result of a null intensity. The isothermalsurface that is obtained when the antenna is equipped with choke 11 isindicated by the continuous curve 10 of FIG. 2.

[0015] Actually, as it can be seen, the introduction of the choke 11causes a substantial increase of the diameter of the antenna, and thenof needle 1, thus limiting its applications when a minimum invasiveoperation is required, such as for example in out-patient's departments,in the repeated treatments, etc.

[0016] For manufacturing reasons, and because of the resistance limitsof the material, the radial dimension of the choke cannot be reducedunder certain limits.

[0017] Furthermore, in case of change of the dielectric characteristicsof the medium caused by the variation of the temperature during thetreatment, or in case of variation of the frequency of the antenna, aswhen a microwave generator with adjustable frequency, the choke cannotbe lengthened or shortened, in order to be always about a fourth of thewavelength long. The impedance of the existing chokes is thereforefixed, whereby the elimination of the returning wave can not be totallyeffective when the operating temperature is changed.

[0018] An application of hyperthermia is, furthermore normallyassociated to a measure of the local temperature. In fact, it isnecessary for measure the heating temperature of the cancer lesions, orother lesions to treat, to preserve the adjacent healthy tissues and forcontrolling the actual heating power of the antenna.

[0019] Usually, in the operation region a sensor of temperature isinserted (indicated with 20 in FIG. 2) For example, metal thermocouplesare used. However, they cannot be introduced during the delivery ofenergy by the antenna, owing to eddy currents in the metal ofthermocouple, that overheats thus affecting the measure. Furthermore thepresence of a thermocouple changes the distribution of the microwavefield, changing the heating cannot be introduced during the delivery ofenergy by the antenna, owing to eddy currents in the metal ofthermocouple, that overheats thus affecting the measure. Furthermore thepresence of a thermocouple changes the distribution of the microwavefield, changing the heating pattern. Therefore, the temperature measurewith a metal thermocouple must be done with the drawback of stoppingoften the delivery of energy. Alternatively, optical-fibre sensors areknown to have no metal and are not affected by the field or do notperturbate it, but have the drawback of being expensive and fragile. Inboth cases of metal thermocouples or optical-fibre sensors, there is thefurther drawback of introducing a further catheter for guiding thesensor into the operation region.

SUMMARY OF THE INVENTION

[0020] It is object of the present invention to provide an co-axialmicrowave antenna for applications in medicine and surgery that isprovided with trap, or choke, for blocking the backwards propagation ofthe reflecting wave towards the generator, wherein a miniaturisation ofthis trap with respect to the prior art is possible, in order to allowthe use for minimally invasive applications.

[0021] It is another object of the present invention to provide anantenna that, in case of variation of the medium wavelength, allows thechoke to be lengthened or shortened for a more correct operation.

[0022] It is a further object of the present invention to provide anantenna that allows a measure of temperature operation region.

[0023] It is a further object of the present invention to provide amethod for the production of a such an antenna that allows thisminiaturisation with simple construction.

[0024] These and other objects are achieved by the antenna according tothe present invention, that can be inserted in a metal needle necessaryfor the introduction of the antenna in the target tissue, the antennahaving:

[0025] an inner conductor,

[0026] a dielectric layer that coats the inner conductor for all itslength,

[0027] an external conductor that covers coaxially the dielectric layerexcept from an end portion,

[0028] a choke mounted outside the external conductor near the endportion, the choke comprising a co-axial conducting portion of diameterhigher than the external conductor,

[0029] a conducting collar for connecting the co-axial conductor to theexternal conductor, the conducting collar being arranged along theco-axial conducting portion opposite to the end portion,

[0030] the characteristic of the antenna being that the coaxialconducting portion of the choke consists in the metal needle same.

[0031] Advantageously, the collar is in sliding contact with the needle,whereby the length of the choke can be changed.

[0032] Preferably, next to the collar and adjacent to the end portion,the antenna has a plastics sheath that is a dielectric layer in thechoke.

[0033] The sheath can be of antiadhesive material and of length thatprotrudes from the needle, preventing the outer portion to adhere totissues during the high temperature heating treatment.

[0034] Advantageously, a thermocouple is provided put through the chokeand the sheath, said thermocouple being in contact with the externalconductor of said co-axial tube and having the sensitive end that comesout from said sheath protruding into the zone of feed of the antenna.

[0035] According to another aspect of the invention, a method ofconstruction of a choke with variable length on a co-axial antenna, theantenna being inserted in a metal needle necessary for the introductionof the antenna in the target tissue, the antenna having:

[0036] an inner conductor,

[0037] a dielectric layer that coats the inner conductor for all itslength,

[0038] an external conductor that covers coaxially the dielectric layerexcept from an end portion,

[0039] the characteristic being that of providing on the antenna aconducting collar near the end portion, whereby the conducting collarslides in the metal needle.

[0040] Preferably, next to the collar adjacent to the end portion aplastics sheath is arranged on the antenna that is a dielectric layer inthe choke.

[0041] The sheath and the dielectric layer of the antenna areadvantageously of PTFE.

[0042] The collar can be made of metal welded to the external conductorof the antenna.

[0043] According to a further aspect of the invention an antenna forhyperthermia, that can be inserted in a metal needle necessary for theintroduction of the antenna in the target tissue, comprises:

[0044] an inner conductor,

[0045] a dielectric layer that coats the inner conductor for all itslength,

[0046] an external conductor that covers coaxially the dielectric layerexcept from an end portion,

[0047] a choke mounted outside the external conductor near the endportion, the choke comprising a co-axial conducting portion of diameterhigher than the external conductor,

[0048] the characteristic being of providing a thermocouple put throughthe choke, said thermocouple being in contact with said externalconductor and having the sensitive end that comes out from said chokeand protrudes into the zone of feed of the antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Further characteristics and advantages of the interstitialmicrowave antenna and of the method for its production, according to thepresent invention, will be made clearer with the following descriptionof an embodiment thereof, exemplifying but not limitative, withreference to the further attached drawings wherein:

[0050]FIG. 3 shows a sectional axial view of an antenna secondinvention;

[0051]FIG. 4 shows an exploded view of the antenna of FIG. 3;

[0052]FIG. 5 shows the antenna of FIG. 3 to which a thermocouple isadded that crosses the choke and protrudes into the zone of feed.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0053] In FIG. 3 an axial cross section is shown of an antenna accordingto the invention inserted in a metal tube that consists in an biopsyneedle 1, for example a 14 Gauge, outer diameter=mm 2.1 needle.

[0054] The antenna, in its active portion at the right of the figure, isa radiating dipole or monopole. More precisely, the antenna is formed bya co-axial tube having an external conductor 2, by a dielectric layer 3and by a centersl conductor 4 immersed in the dielectric layer 3 thatthe insulates from the external conductor 2. The external conductor 2,as well known, end point indicated with 7, which is the feeding pointthe active portion of the antenna, said feed in gergo tecnico, where theemitted power, normally, is maximum.

[0055] According to the invention, is provided a plastics sheathinsulating 5 and a collar metal 6. More precisely, this result can beobtained:

[0056] saldando on the external conductor of the antenna 2 the metalcollar 6 in a predetermined position,

[0057] by arranging the sheath 5 of plastics that coats the externalconductor 2 in the portion that goes from feed 7 up to collar 6,

[0058] using the wall inner of the same metal needle 1 wherein theantenna is inserted for containing and guiding the collar 6 and thesheath 5, in particular, the collar 6 being in electrical contact withthe wall inner of the metal needle 1.

[0059] The invention allows to make in a easy and not expensive way aminiaturized microwave antenna equipped with choke, suitable for localtreatment of deep lesions in medicine and surgery. In fact, incombination with the metal needle 1, the collar 6 and the sheath 5 allowto obtain a choke of variable length and made reducing to the minimumthe increase of the outer diameter of the antenna.

[0060] Like the choke of FIG. 2, in fact, in FIG. 3 the wave 13 isreflected back starting from the feed 7 and runs the external surface ofthe antenna, enters the choke formed between the needle 1 and theexternal conductor 4, reflects itself on the collar 6 in short circuitand, after a total λ/2 path is again at the inlet of the choke in phaseopposition with respect to the wave at the inlet, obtaining a nullintensity. The variation of wavelength in case of temperature rise, orother cause, can be corrected by varying the position of the collar 6with respect to needle 1, so that the choke is always λ/4 long. Within acertain range the variation of impedance of the antenna during theoperation can be compensated in the same way changing the length of thechoke and then of the portion between the choke and the feed.

[0061] The isothermal surface that is obtained when the antenna isequipped with the choke according to the invention is shown also in thiscase by the dashed curve axial tube of the antenna, and the inner wall 1of the needle guide through which the antenna is inserted.

[0062] More precisely, the metal collar 6 keeps the electrical contactwith the inner wall of the needle guide 1 and is thus a mobile by-pass.

[0063] Sheath 5 has, then, the following functions:

[0064] is a co-axial wave guide λ/4 long supplying an effective chokefor the antenna,

[0065] it is a centering element for the sliding in the antenna needle,

[0066] in the portion outer to the choke the adhesion of tissues isavoided during the high temperature heating treatment, and it does notallow their contact with metal surfaces different from the needle guidewithin which the antenna slides.

[0067] With reference to FIG. 5, according to a different embodiment ofthe invention, a thermocouple is provided 21 put through the collar 6and the sheath 5 that forms the choke. Thermocouple 21 is in contactwith the external conductor 2 of the co-axial tube that forms theantenna, and has its sensitive end 22 that comes out from the sheath 5protruding into the zone 10 of feed of the antenna. The other end ofthermocouple is connected to the a temperature measuring instrument notshown by means of a plug 23.

[0068] According to the invention thermocouple 21 does not affects theoperation of the antenna. In fact, the thermocouple is integral to themetal of the outer coaxial conductor 2. Therefore, thermocouple 22 ispractically shielded.

[0069] Thermocouple 21 can be a common metal thermocouple, formed by ametal sheath in which are metal different conductors are joined at thesensitive end 22. Such a metal thermocouple not much expensive, muchless than in optical fibre sensors (for example fluoride-opticalsensors).

[0070] A further advantage of thermocouple 21 is that of an externalthermocouple has not to be inserted through an additional catheter topart and, especially, the measure can be made directly in the operationregion during the feeding delivery that produces the hyperthermia.

[0071] Thermocouple 21 can be put also in hyperthermia antennasdifferent from that shown in figures from 3 to 5.

[0072] The foregoing description of a specific embodiment will so fullyreveal the invention according to the conceptual point of view, so thatothers, by applying current knowledge, will be able to modify and/oradapt for various applications such an embodiment without furtherresearch and without parting from the invention, and it is therefore tobe understood that such adaptations and modifications will have to beconsidered as equivalent to the specific embodiment. The means and thematerials to realise the different functions described herein could havea different nature without, for this reason, departing from the field ofthe invention. It is to be understood that the phraseology orterminology employed herein is for the purpose of description and not oflimitation.

1. Co-axial microwave antenna for applications interstitial,percutaneous, laparoscopic, endoscopic and intrasurgery in medicine andsurgery, in species for acute hyperthermia in oncology, that can beinserted into a metal needle for introduction into a target tissue, saidantenna having: an inner conductor, a dielectric layer that coats theinner conductor for all its length, an external conductor that coverscoaxially the dielectric layer except from an end portion, a chokemounted outside the external conductor near said end portion, said chokecomprising a co-axial conducting portion of diameter higher than theexternal conductor, a conducting collar for connecting said axialconductor to the external conductor, said conducting collar beingarranged along said co-axial conducting portion opposite to said endportion, characterised in that the co-axial conducting portion of thechoke consists in said metal needle.
 2. Antenna second claim 1, whereinsaid collar is in sliding contact with said needle, whereby the lengthof the choke can be changed.
 3. Antenna second claim 1, wherein next tosaid collar adjacent to said end portion a plastics sheath is providedthat is a dielectric layer in the choke.
 4. Antenna second claim 1,wherein said sheath is of antiadhesive material and has a length thatprotrudes from the needle, preventing the outer portion to adhere totissues during the high temperature heating treatment.
 5. Antenna secondclaim 1, wherein a thermocouple is provided put through the collar andthe sheath that forms the choke, said thermocouple being in contact withthe external conductor of said co-axial tube and having the sensitiveend that comes out from said sheath protruding into the zone of feed ofthe antenna.
 6. A method of construction of a choke with variable lengthon a co-axial microwave antenna for interstitial, percutaneous,laparoscopic, endoscopic and intrasurgery applications in medicine andsurgery, in species for acute hyperthermia in oncology, said antennabeing inserted into a metal needle necessary for introduction into atarget tissue, said antenna having: an inner conductor, a dielectriclayer that coats the inner conductor for all its length, an externalconductor that covers coaxially the dielectric layer except from an endportion, characterised in that of providing on said antenna a conductingcollar near said end portion, whereby said conducting collar slides insaid metal needle.
 7. Method according to claim 6, wherein next to saidcollar adjacent to said end portion a plastics sheath is provided on theantenna that is a dielectric layer inner to said choke.
 8. Methodaccording to claim 6, wherein said sheath and the dielectric layer ofthe antenna are of PTFE.
 9. Method according to claim 6, wherein saidcollar is made of metal welded to the external conductor of the antenna.10. A antenna for hyperthermia, that can be inserted in a metal needlenecessary for the introduction of the antenna in the target tissue,comprising: an inner conductor, a dielectric layer that coats the innerconductor for all its length, an external conductor that coverscoaxially the dielectric layer except from an end portion, a chokemounted outside the external conductor near the end portion, the chokecomprising a co-axial conducting portion of diameter higher than theexternal conductor, characterised in that a thermocouple is provided putthrough the choke, said thermocouple being in contact with said externalconductor and having the sensitive end that comes out from said chokeand protrudes into the zone of feed of the antenna.